The present invention relates to a ceramic heater and a glow plug having the ceramic heater.
Hereinafter, the term xe2x80x9cfrontxe2x80x9d refers to a heating end side with respect to the axial direction of a rod-shaped ceramic heater, and the term xe2x80x9crearxe2x80x9d refers to a side opposite the front side.
A glow plug is widely used, which comprises a cylindrical metallic shell, a rod-shaped ceramic heater disposed in the metallic shell with a front end portion thereof protruded from the metallic shell, a central electrode partly disposed in a rear portion of the metallic shell and connected to power source, and a metallic lead through which the ceramic heater and the central electrode are electrically connected to each other. In such a structure, the ceramic heater is externally energized through the central electrode and the lead.
Conventionally, the ceramic heater and the lead are connected to each other by the following methods (1) to (3):
(1) a front end portion of the lead is coiled, and a heater terminal exposed at a rear end of the ceramic heater is inserted into and brazed to the coiled front end portion of the lead, as disclosed in Japanese Laid-Open Patent Publication No. 10-205753;
(2) a metallic connecting cap is brazed to a rear end of the ceramic heater so that the connecting cap covers both of a rear end surface and an outer circumferential surface of the ceramic heater, and a front end portion of the lead is brazed to the connecting cap, as disclosed in Japanese Laid-Open Patent Publication Nos. 4-268112 and 62-141423 and Japanese Patent Publication No. 60-30608; and
(3) a front end portion of the lead is embedded in a rear end of the ceramic heater, as disclosed in Japanese Laid-Open Patent Publication No. 2000-356343.
However, there are some problems in the above conventional methods (1) to (3).
It has been increasingly demanded to make the glow plug compact in size in order to provide a multivalve diesel engine and to achieve weight reductions of engine parts. In the method (1), however, the coiled end portion of the lead takes up radial space around the rear end of the ceramic heater. Thus, such a demand cannot be always satisfied because of the radial space for the coiled end portion of the lead, even when the diameter of the ceramic heater is made smaller. Further, there arises the possibility of a short circuit upon placement of the coiled end portion of the lead in a very small clearance between the metallic shell and the ceramic heater. The demand to make the glow plug compact in size cannot be always satisfied either in the method (2), because the connecting cap takes up radial space around the ceramic heater. In addition, the ceramic heater is strongly acted upon by a thermal stress through the connecting cap, whereby the ceramic heater tends to become cracked. In the method (3), the front end portion of the lead has to be formed as a sintered member separately, thereby resulting in much expenses in time and effort for production. Further, the joint surface between the ceramic heater and the lead tends to be insufficient to attain a good joint strength.
It is therefore an object of the present invention to provide a ceramic heater which can be produced easily and, when applied to a glow plug, can reduce the risk of a short-circuit and maintain a proper joint between the ceramic heater and the lead in repeated cycles of heating and cooling while allowing the glow plug to become compact in size.
It is also an object of the present invention to provide a glow plug using such a ceramic heater.
According to a first aspect of the present invention, there is provided a ceramic heater comprising: a rod-shaped heater body having an insulating ceramic substrate, a heating resistor embedded in a front end portion of the ceramic substrate and an electric conductor embedded in the ceramic substrate with a front end portion thereof electrically connected to the heating resistor and a rear end portion thereof exposed at a rear end surface of the heater body; and a lead-out member having a front surface joined to part of the rear end surface of the heater body via a metallic layer so as to cover the exposed rear end portion of the electric conductor and to be kept from covering an outer circumferential surface of the heater body.
According to a second aspect of the present invention, there is provided a glow plug comprising: a ceramic heater provided with a rod-shaped heater body and a lead-out member, the heater body having an insulating ceramic substrate, a heating resistor embedded in a front end portion of the ceramic substrate and an electric conductor embedded in the ceramic substrate with a front end portion thereof electrically connected to the heating resistor and a rear end portion thereof exposed at a rear end surface of the heater body, the lead-out member having a front surface joined to part of the rear end surface of the heater body via a metallic layer so as to cover the exposed rear end portion of the electric conductor and to be kept from covering an outer circumferential surface of the heater body; a metallic sleeve circumferentially surrounding the heater body with a front end portion of the heater body protruded from the metallic sleeve; and a metallic shell fitted onto a rear end portion of the metallic sleeve and having a mounting portion on an outer circumferential surface thereof so as to mount the glow plug in a cylinder head.
According to a third aspect of the present invention, there is a ceramic heater comprising: a rod-shaped heater body having an insulating ceramic substrate, a heating resistor embedded in a front end portion of the ceramic substrate, and a pair of first and second electric conductors embedded in the ceramic substrate with front end portions thereof electrically connected to the heating resistor and rear end portions thereof exposed at a rear end surface of the heater body; and first and second lead-out members having front surfaces joined to parts of the rear end surface of the heater body via metallic layers so as to cover the exposed rear end portions of the first and second electric conductors, respectively, and to be kept from covering an outer circumferential surface of the heater body.
According to a fourth aspect of the present invention, there is provided a glow plug comprising: a ceramic heater provided with a rod-shaped heater body and a pair of first and second lead-out members, the heater body having an insulating ceramic substrate, a heating resistor embedded in a front end portion of the ceramic substrate, and a pair of first and second electric conductors embedded in the ceramic substrate with front end portions thereof electrically connected to the heating resistor and rear end portions thereof exposed at a rear end surface of the heater body, the first and second lead-out members having front surfaces joined to parts of the rear end surface of the heater body via metallic layers so as to cover the exposed rear end portions of the first and second electric conductors, respectively, and to be kept from covering an outer circumferential surface of the heater body; a metallic sleeve circumferentially surrounding the heater body with a front end portion of the heater body protruded from the metallic sleeve; and a metallic shell fitted onto a rear end portion of the metallic sleeve and having a mounting portion on an outer circumferential surface thereof so as to mount the glow plug in a cylinder head.